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Originally Posted by YendorLootmonkey
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One interesting (yet depressing) theory, and I can't remember what it's called, is that the reason we have not been visited by intelligent life after billions of years during which other civilizations could have advanced, is that there is always some sort of point at which civilizations wipe themselves out before getting to the point where they can perform interstellar travel. Part of this theory is that before they get to that point, civilizations wipe themselves out due to:
a) depletion of resources (via war, famine, disease, etc), or
b) planetary catastrophe during technological/scientific advancement.
Now the most interesting thing about b) is that it has been postulated that, if finding the Higgs-Boson particle is a pre-requisite for the understanding of physics necessary to develop interstellar travel, every civilization could advance to the point where they search for Higgs-Boson and create their equivalent of the Large Hadron Collider, at which point something goes horribly wrong and they create a black hole that devours their entire planet.
Taking that theory even further, it could be theorized that at least some black holes that we have detected are the results of past civilizations getting to that point and then inadvertantly creating a black hole on themselves.
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http://en.wikipedia.org/wiki/Fermi_paradox
Its some pretty depressing stuff, but not all of it is that bad. One of the theories that seems to make sense is a civilization has a relatively short "radio age" where they are broadcasting signals that we can pick up. Already we are moving away from radio waves, replacing analogue tv with digital, and internet tv for example. We've only been broadcasting radio waves for maybe 100 years now, and even if it takes another 100 or more before they're gone, its still a drop in the bucket when you're talking about the lifespan of a civilization.
Another not depressing theory is that we're just too far away from any aliens to communicate. We're sending a signal to stars 24,000 light years away. Assuming that there are aliens there, what are the chances that they are still listening? Even if they are listening, we would have to wait another 24,000 years to hear a response, and much longer if we were to expect some kind of visit or other contact.